Influenza viruses cause annual influenza epidemics and occasional pandemics, which pose a significant threat to public health worldwide. Seasonal influenza infection is associated with 200,000-500,000 deaths each year, particularly in young children, immunocompromised patients and the elderly. Mortality rates typically increase further during seasons with pandemic influenza outbreaks. There remains a significant unmet medical need for potent anti-viral therapeutics for preventing and treating influenza infections, particularly in under-served populations.
There are three types of influenza viruses: types A, B and C. The majority of influenza disease is caused by influenza A and B viruses (Thompson et al. (2004) JAMA. 292:1333-1340; and Zhou et al. (2012) Clin Infect. Dis. 54:1427-1436). The overall structure of influenza viruses A, B and C is similar, and includes a viral envelope which surrounds a central core. The viral envelope includes two surface glycoproteins, Hemagglutinin (HA) and neuraminidase (NA); HA mediates binding of the virus to target cells and entry into target cells, whereas NA is involved in the release of progeny virus from infected cells.
The HA protein is responsible for the binding to the host cell receptor as well as fusion of viral and host cell membranes and is the primary target of protective humoral immune responses. The HA protein is trimeric in structure and includes three identical copies of a single polypeptide precursor, HA0, which, upon proteolytic maturation, is cleaved into a metastable intermediate containing a globular head (HA1) and stalk region (HA2) (Wilson et al. (1981) Nature. 289:366-373). The membrane distal “globular head” constitutes the majority of the HA1 structure and contains the sialic acid binding pocket for viral entry and major antigenic domains. The membrane proximal “stalk” structure, assembled from HA2 and HA1 residues, contains the fusion machinery, which undergoes a conformational change in the low pH environment of late endosomes to trigger membrane fusion and penetration into cells. The degree of sequence homology between influenza A subtypes is smaller in the HA1 (34%-59% homology between subtypes) than in the HA2 region (51%-80% homology).
Influenza A viruses can be classified into subtypes based on genetic variations in hemagglutinin (HA) and neuraminidase (NA) genes. Serologically, influenza A can be divided into 18 HA subtypes which are further divided into two distinct phylogenetic groups: group 1 (subtypes H1, H2, H5, H6, H8, H9, H11, H12, H13, H16, H17 and H18) and group 2 (subtypes H3, H4, H7, H10, H14, and H15). Currently, in seasonal epidemics, influenza A H1 and H3 HA subtypes are primarily associated with human disease, whereas viruses encoding H5, H7, H9 and H10 caused sporadic human outbreaks due to direct transmission from animals. In contrast to influenza A viruses, influenza B viruses are restricted to human infection and influenza B viruses are not divided into subtypes based on the two surface glycoproteins. In fact, until the 1970s, influenza B viruses were classified as one homogenous group. However, through the 1970s, the influenza B viruses started to diverge into two antigenically distinguishable lineages which were named the Victoria and Yamagata lineages after their first representatives, B/Victoria/2/87 and B/Yamagata/16/88, respectively. (Biere et al. (2010) J Clin Microbiol. 48(4):1425-7; doi: 10.1128/JCM.02116-09. Epub 2010 Jan. 27). Both Yamagata and Victoria lineages contribute to annual epidemics. Although the morbidity caused by influenza B viruses is lower than that associated with influenza A H3N2, it is higher than that associated with influenza A H1N1 (Zhou et al. (2012) Clin Infect. Dis. 54:1427-1436).
Neutralizing antibodies elicited by influenza virus infection are normally targeted to the variable HA1 globular head to prevent viral receptor binding and are usually strain-specific. Broadly cross-reactive antibodies that neutralize one or more subtype or lineage are rare. Recently, a few antibodies have been discovered that can neutralize multiple subtypes of influenza A viruses in both group 1 and 2 (Corti et al. (2011) Science 333(6044):850-856, Li et al. (2012) PNAS 109(46):18897-18902, Dreyfus et al. (2012) Science 337(6100):1343-1348, and Nakamura et al. (2013) Cell Host and Microbe 14:93-103), or influenza B viruses of both lineages (Dreyfus et al. (2012) Science 337(6100):1343-1348 and Yasugi et al. (2013) PLoS Path 9(2): e1003150. doi: 10.1371/journal.ppat.1003150), although most have limitations in breadth of coverage, resistance profile, or potency. Only one antibody has been described to bind to both influenza A and B HA proteins, although this antibody does not functionally neutralize influenza B viruses or attenuate disease when given therapeutically (Dreyfus et al. (2012) Science 337(6100):1343-1348). To date, there are no available antibodies that broadly neutralize or inhibit a broad spectrum of influenza A and B virus infections or attenuate diseases caused by influenza A and B virus. Therefore, there is a need to identify new antibodies that protect against multiple influenza viruses.